Radio navigation systems



'July 22, 1958 w. J. o'BRlEN ETAL 2,844,816

RADIO NAVIGATION SYSTEMS Filed March '7, 1955 4 sheets-sheet'l FY'AAMM I,mwN

July 22 1958 A W. J. O'BRIEN ETAL 2,844-,816

RADIO .NAVIGATION SYSTEMS Filed March 7, 1955 4 Sheets-Sheet 2 15A-AWijill PowER Powfn Powsk PHASE PHASE PHASE 1,5 canino. conlol amiko IFPips 17 15 if( PlPPEQ. A' l' Alum?. K D'mvo :0F loF -ANI". .MR

9F J AM? PHASE, AMV f 'Y DlscizuMmM-OR/ n PHASE moncArosz @.5 g2. f ,f4f5 fr s: 2f 6 PHASE SWITCH Mulan) l HMumPuecz ,DuscQIMwATo f. F /w 50aal 6F /"57M v A6155. 05C. f L +6 "BlvlDEZf AAIIEP l SwlTcH AMI.

. Y 4,? a@ ai 5, 455 am sappen; S2 65 REFEREM SHIFT #ne a G, V4/5eplpnera/ p I ,rrae/vi f Filed March "f, 1955 y W. J. O'BRIEN ETAL RADIONAVIGATION SYSTEMS 4 Sheets-Sheet 3 July 22, 1958 W. J. O'BRIEN ET ALRADIO NAVIGATION SYSTEMS Filed March '7, 1955 4 Sheets-Sheet 4COINCIDENCE DE'TC, TOR.

' Unite f tions, given in radians of angle, nlwot-l-al,

2,844,816 RADIO NAVIGATION SYSTEMS William Joseph OBrien and vHaroldGeorge" Hawker,

London, England, assignors to The DeccaV Record Conipany Limited,London, England, a British company Application March 7, 1955, Serial No.492,592 Claims priority, application Great Britain March 8, 1954 18Claims. (Cl. 343-105) This invention relates to hyperbolic radionavigation systems 1n which, at a mobile receiver, a phase comparison 1smade between signals received from spaced transmittlng stations. Y

In hyperbolic radio navigation systems, a position line may be obtainedby determining the difference in 'propagatlon tlrne of signals emittedfrom two spaced stations. The present invention relates moreparticularly to hyperbohc radio navigation systems in which signals aretransmitted alternately or in sequence from the stations, the radiatedsignals being synchronised continuous wave signals of harmonicallyrelated frequencies.

According to this invention, in a hyperbolic radio navigatron system inwhich the position line is determined by indicating the difference inthe time of propagation to a receiver of signals emitted from twostations vin known spaced geographical position, three signals areemitted from a first station satisfying the-respective phasecondin2w0t+a2 and (n1|1)w0t|a3, 'and three'signals areemitted fromV thesecond station satisfying the respective phase' conditionsnlwot-l-al-l-k, nzwot-l-az-l-k and (n1+1)w0t|a3'+k, where nl and n2 areintegers, wo isV 21rfmultiplied 'by a fundamentalfrequency in cyclespersecond, t is time inseconds and al, a2, a3 and k are constants, thetransmissions being switched in sequence such that theY signals ofv eachfrequency are transmitted from one Ystation -during intervals of thetransmissions of the same frequency from the other station, and thetransmission being distinctively altered periodically for synchronisingswitching means at Vthe receiver with the switching of thevtransmissionsand wherein the receiver comprises means Vfor receiving theradiated'signals, switching means which are synchronised by thedistinctive alteration in transmission and which are arranged toseparate the received signals from the 'two transmitters, and a phasedifference indicator for indicating the difference in time ofpropagation which indicator has'a recurrence cycle of time differenceindication equal, in seconds, to the reciprocal of the fundamentalfrequency, said indicator being operated by a fine control and' a coarsecontrol, said fine control being dependent on a pair of signals derivedfrom locally generated non-inter` rupted signals, one of which lisphase-controlled by only one of the signals received from said Vfirststation and the other byv only one of the signals received from saidsecond station and the coarse control being dependenton six receivedsignals derived from the threedilferent frequencies transmittedfrom eachof the stations.` This ar- 4 rangement rpermits of the fullest amountofi-nformation to be obtained from the limited vnumber of differenttransmitted frequencies, thus enabling this information Vto be utilisedto reduce to a minimum boththeinherent amn4 biguities in time or phaseindication and also the ambiguity resulting from phase shifts due toreception lof signals reflected from the ionosphere; v f

In addition to providing -a iinewcontrol and a'coarse control, it wouldbe possible also'to-pro'vide one or more intermediate'controls. Suchanintermediate con- States Patent() trol may be eected using only thewise stated, it will be Vassumed that a single .arrangement employingonly three frequencies is employed.

In one arrangement of the invention,thetwo` locally f generatednon-interrupted signals may be produced in' the receiver by a pair ofoscillators arranged to 'oscillate at the same frequency, whichfrequency is oneV of the radiated frequencies, the two'oscillators beingcontrolled in phase respectively by the phase received from the twostations. Sometimes, however, it may be preferablethat at least one ofthe two locally generated `non-interrupted signals is producedl in thereceiver by an oscillator ar-Y ranged to oscillate at said fundamentalfrequency; :These oscillations at the fundamental frequency may readilybe converted to signals of any of the multiples of the fundamental by aharmonic generator, such as, for example, a pipper such as that shownand described in the W. J. OBrien Patent No. 2,524,677,7issued October'3;, 1950, which produces pulse `signals of extremely short duration at afixed pulse recurrence'.frequencyn'Such signals are characterizedby'many strongharmonicsain xed phase relationship. This oscillatormayrthus corr-y veniently serve also .as areference signal generator.Y

At each station thevarious diiferent-nequencies may be radiatedsimultaneously.l Thesignals'mustr'berradi- Y, ated intermittently from*the two 'stations' so that fthey may be separately received at thereceiver. :Suclrlnter-A mittent radiation of a plurality of differentVfrequencies at each of the two stations may conveniently loccurv duringintervals in a normal transmission Yin'which signals of differentfrequencies Vare radiatedsimultaneosly from the two stations. Thisintermittent radiation may `'be Varranged, for example, to serve as laneidentication'transmissions in a system such as is' described in `Britishpatent specification No. 656,108 in which transmissions prviding normalposition fixing are periodically interrupted to enable signals providinga coarser positionxing to be transmitted. As described in thatspecification, during the normal transmissions a single signal mayberadiated from each -station and conveniently the frequency of each ofthe single signals is one of the aforementionedk frequencies n1, n2 ornl-l-l multiplied by the yfundamental frequency. At the receiver `thesimultaneously'received signals of differentV frequencyfrom the first"lstation lmay be combined to produce a signalof'the fundamental ire'-quency, which signal of the fundamental frequencyris used to phase-locka frequency rdivider coupledvtodivida to the fundamental frequency, thatone ofthe locally generated non-interrupted signals which is*phasei'controlled by the first station-thereby providing a'rst output ofsaid fundamental frequency and the simultaneously received signals vvofditer'entfrequency from .thesecond station may be combined to produceas'ignal of the fundamental frequency, 'wh'ich signal Yof theAfundamental free Y quency is used to phase-lock va secondV frequencydivider Y coupled to divide, to the fundamental frequency, thei'fsec.ond of the locally generated non-.interrupted signalswhich are phasecontrolled by the Ysecondjstationithereby'prol` viding .asecond Youtputof.. said?fundamentalreqencyx threefreqenciesifrom each station or oneormore additional frequencies may ne yand a coarse control Yare pro--and in which Ythe two locally generated non-interrupted 'signals areproduced in the receiver by a pair of oscillators arranged to oscillateat the same frequency, which frequency is one of the radiatedfrequencies, the two oscillators being controlled in phase respectivelyby the signals received from the two stations, there may be provided atthe receiver a pair of dividers associated respectively with the twooscillators each divider being arranged to divide the oscillator outputfrequency to a frequency equal to the difference between n1 and n2multiplied by the fundamental frequency. Each of the dividers may thenbe phase locked by a beat signal obtained by combining two signals, oneof which is the output of the associated oscillators and the other ofwhich is that one of the three signals received from the transmittercontrolling the associated oscillator which dilfers in frequency fromthe oscillator frequency by an amount equal to the frequency of thedivider output. The two divider outputs might be compared in phase toprovide a relatively coarse pattern. However, to make the fullest use ofthe radiated signals it is preferable that the oscillators at thereceivers should be controlled by the received signals of n1 or n1+1multiplied by the fundamental frequency and that there should beprovided a further pair of frequency dividers arranged respectively todivide the outputs of the rst pair of dividers to the fundamentalfrequency. Each of the second pair of dividers may then be phase lockedby a beat signal obtained by combining two signals, one of which is theoutput of the associated oscillator and the other of which is that oneof the three signals received from the transmitter controlling theassociated oscillator which differs in frequency from the oscillatorfrequency by an amount equal to the fundamental frequency, the twooutputs from the further pair of dividers being used to effect thecoarse control of the phase difference indicator. By this arrangement aphase comparison may be made which is effectively a phase comparison ofsignals of the fundamental frequency from the two transmitters.

Although in the arrangements described aboveV only two stations havebeen considered which will provide a single position line, as is wellknown in the art of hyperbolic radio navigation, two position lines andhence a position x may be obtained by providing either two pairs ofstations or by providing a third (and possibly one or more furthertransmitting stations), all the stations in the latter case beingsynchronised in phase. Thus there may be provided, in the system of thepresent invention, a third transmitting station radiating signalssatisfying the respective phase conditions, given in radians of anglewhere k1 is a constant, the transmissions being switched in sequencesuch that the signals of each frequency are transmitted from one stationduring intervals of the transmissions of the same frequency from theother stations. With this transmitting arrangement it is possible toobtain two position line indications, that is to say'to obtain aposition tix and for this purpose the receiver may be provided with twophase dierence indicators each operated by a fine control and a coarsecontrol, as previously set forth, one indicator being arrangedtoindicatethe difference in time of propagation of signals from the rst and secondstations and the other being arranged to indicate the dierence in timeand propagation of the signals from the third station.

The following is a description of a number of embodiments of theinvention reference being made to the accompanying drawings in which:

Figures l, 2 and 3 are diagrams illustrating wave forms obtained bycombining signals of harmonically related frequencies;

Figure 4 is a block diagram illustrating a transmitting system;

Figure 5 is a block diagram illustrating a multiplex transmitter as usedin the system of Figure 4;

Figure 6 is a table showing time schedules of transmissions, and

Figures 7, 8 and 9 are block diagrams of three forms of receivingapparatus.

The present invention makes use of the information that can be obtainedfrom combining a number of continuous wave signals of harmonicallyrelated frequencies which are in a fixed phase relationship. Figure 1illustrates the combination of two waves which are the eighth and ninthharmonics of a fundamental frequency, the drawing showing a time periodequal to a half cycle of the fundamental frequency as is apparent frominspection of the envelope of the wave shown. From this envelope it ispossible to determine the phase of the fundamental frequency. Increasingthe number of signals combined in this manner gives a marked improvementin the definition of the phase of the fundamental. Thus in Figure 2there is shown a combination of the sixth, eighth and ninth harmonicsand in Figure 3 there is shown the combination of the fifth, sixth,eighth and ninth harmonies. In Figures 2 and 3 there is a marked andclearly defined peak which can be used in particular to define the phaseof the fundamental frequency. The present invention makes use of thisfurther information regarding the phase of the fundamental frequencywhich can be obtained from the use of three or more harmonically relatedcontinuous wave signals.

Figure 4 is a block diagram of a master and a slave transmitter for ahyperbolic radio navigation system. At the master station an oscillator1 of frequency 1f feeds a pulse forming circuit 2 which produces shortduration pips at a repetition rate of 1f. These pips are fed into amultiplex transmitter 3 which energises the transmitting antenna 4 withsignals of the required harmonics of lf. At the slave station, thesesignals are received by antenna 5 coupled to a receiver 6, which latterserves to control the phase of pips having a v1f repetition rategenerated by a circuit 7. These pips from the circuit 7 are fed, in asimilar manner to that employed at the master station, to a multiplextransmitter 8 which in turn energises the transmitter 9 with therequired harmonic signals.

The multiplex transmitter used in the master and slave stations is shownin further detail in Figure 5. The pips of frequency lf from the input10 are fed into the inputs of two amplifiers 11 and 12 tuned to thefrequency 12f. The output from amplifier 12 is passed through a combinedmanual and electronic phase control circuit 13 to a power amplier 14,theoutput of which energises the transmitting antenna 15 through afour-position rotary switch 15a connected as shown between the powerampliiers and the antenna 1S. A signal from the antenna 15 is picked upat 16 and is fai intoan amplifier 17 tuned to the frequency 12f which inturn feeds the amplied signal to one input of a phase discriminator 18.The output from the amplifier 11 is applied to the second input of thephase discriminator 18 which serves to compare the phase of these twosignals of frequency l2f and to produce a direct voltage output which isapplied to the electronic phase control circuit 13. The phase shiftsthrough the amplifiers 11 and 17 are adjusted to give zero discriminatoroutput voltage when the inputs to the amplifiers are coupled to a commonsource. By this ar-A f rangement the output phase of Athe radiatedsignal as seen at' the input of the amplifier 14 is maintained in phasewith the plps of 1f frequency from the circuit 10 which is the input tothe amplifier 11. The circuit just described thus serves to maintain thephase relationship of the radiated signals of frequency 12f. Thiscircuitry can be duplicated for any number of harmonics and there areshown in Figure 5 similar circuits for controlling also the phase ofradiated signals of frequency lOf and 91. The radiation of any of theseharmonics may be switched on or off at will provided the antenna ismulti-tuned. lf the antenna isnot multi-tuned, then only one signal maybe radiated at a time and the antenna tuning must be switched toresonate with the applied signal.

Triple tuned transmitting antennae with phase control .Y of the typedescribed are in commercial use anda more detailed description of thisform of phase control of harmonically related radiated signals is givenin the specifications of British Patent Nos. 656,108 and 656,126.However, at very low frequencies, such as for example below 20 kc./s.,the cost of multiple tuning .of a transmitting antenna'becomes almostprohibitive if high power radiation is to be obtained. At such very lowfrequencies, therefore, it may be desirable to limit the radiation fromeach station to one signal at a time and this can be achieved byswitching the signals so that they are radiated in sequence at eachstation.

Figure`6 is a table showing the allocation of time periods suitable fora very low frequency system employing three stations each radiating onthree frequencies and arranged so that each station only radiates onefrequency at a time. It will, of course, be appreciated that differentstations may radiate different frequencies at the same time. In the timeschedule a blank period is shown and this period may be used forsynchronising the switching at the transmitting stations and at thereceivers. The switching rate may be'very slow and might, for example,be controlled by means of a synchronous motor 15b such as is used for asynchronous electric clock. However, many methods of synchronisingswitching at the receiver utilising an alteration. (such as theinterruption provided bythe blank period) of the transmissions from onestation are known in the art.

. A receiver for use with a transmitting system operating on theschedule given in Figure 6 is shown in Figure 7. The antenna 19 picks upall the received signals and feeds them to the inputs of threeamplifiers 2t), 21, 22 which yare tuned respectively to the ninth,twelfth and tenth harmonics of a fundamental frequency f. The output ofamplifier Ztl is ,applied to one input of a phase discriminator 23.During the periods when the 9f Signal is radiated from ythe station A,the other input of discriminator 23 is fed from an oscillator 24 offrequency 97 through the appropriate contact of switch 25 which isdriven by synchronous motor 25a in synchronism with the switching of thetransmissions. The discriminator 23 produces a direct voltage routputdependent on the phase relationship of the two applied signals and thisoutput is applied tooscillator 24 via switch 26 ,so as to adjust theoscil- Vlator in phase in a manner to maintain the phase of theoscillator in synchronism with the output of the amplifier 20.l By thismeans the oscillator 24 produces a continuous signal, that is anon-interrupted signal, which is phase locked to the interrupted signalreceived from the amplifier 20. The oscillator 24 thus provides anoutputWhichfis equivalent to that which would be obtained from the amplifier20 ifqthe signal of frequency 9f were continuously transmitted only fromstation A. A second and a third 9f oscillator 27 and 28 Vare phaselocked in a like manner to' the output of amplifier y20 Vduring thetimes .of transmission of the 9f signal from stations B and Crespectively.

The outputs of frequency 9f from each of the oscillay Ytors 24, 27 and28 are reducedrto a T frequency` of `3f by Ythe frequency ldividers 29,30 and 31 respectively. These' 6 t signals of frequency 3f are againdivided downtoa frequency of 1f by the dividers 32, 33 and 34.

A mixer 35 derives a signal of frequencyV fronithe I beat note resultingfrom lmixing the 12j. output o f amplifier 21 with the 9j signal fromthe oscillator 2 8 via switch 36 during the period when the 12f input ofamplifer 21 is received from C station. Thisf output from the mixer 35is applied to the C station divider31Y via the switch 37. By this meansthel divider 31 is properly notched, that isit is under control by aparticular one of each group of three cycles of the 91 input. TheYdividers 29 and 30 Vare notched in the same manner at the appropriatetime periods, that is to say the second and fourth periods on the tableinl Figure 6 when the l2f sigoutput from mixer 38 is applied to the Bstation divider 33 via the switch 40. By this arrangement the divider 33is notched, that is it is controlled by a particular one of each groupof three pulses in the divider 30, which means it is under control by aparticular one cycle of each group of nine cyclesk from the oscillator27. lThe dividers 32 and 34 are notched in a similar manner at theappropriate time periods, that is to say the third and second periodsrespectively of the schedule shown'in Figure 6. A synchronizer 40a maybe connected to control the motor 25a, the synchronizer 40a. beingconnected to theoutputs of, amplifiers 2t), 21, and 22` and, arrangedVto respond to the cessation of the lOf signalata timefwhen all threefrequencies are simultaneously receivedV (this corresponds to the end ofthe rst period lof Fig. 6). Such coincidence detectors are well known.

It will be seen that the arrangement thus far described providescontinuous 9f, signals at the receiver -corresponding to theinterrupted9f signals from "each of the three stations, the continuouss'ignals'being phase locked to the received interrupted signals.There'isalso pro-` vided a lf signal for eachstation which istlieequivalent of a 1f signal transmitted in the form of a 9f'sig'nal iuwhich an individual cycle is identiable in each group of nine cycles. f

A phase difference indicator 4 1 provides affine indication of thedifference in time of propagation from the A and B stations to thereceiver.A The indicator 4 1 is fed from the outputs of oscillators 24and 2.7. Another phase difference indicator 42 is coupled tothe lfNoutputs of the dividers 32 and 33 to provide a coarse indication whicheliminates or reduces the ambiguities of the indicator 41 and so extendsthe range of indication of the indicator 41. It will be noted that thesetwo indicators 41 and 42 together indicate one position line in thepattern of hyperbe referenced, that is the relevant phase differencesinl the various stages in the receiver must be adjusted to known values.A lgenerator 45 vof pips having a recurrence frequency of 1f is providedfor this purpose, the pips providing a series of harmonics ofthefundamental in fixed phase relation to form a phase Standard forreferencing. Y i

In a transmitting systemfor use with vthe receiver of Figure 7, thereceivers at the slave stations, that is the receiver 6 in Figure 4, maybe made similar to part of the receiverjust described with reference toFigure 7. The 1f pips of the circuit 7 in Figure 4 would be suppled'bythepart of the receiver corresponding to the divider 32 ofFigure 7. Inusing the receiver` of VFigure.r 7 as the receiver for aV slave station,the oscillators 27 and 28 and their associated dividers 30, 31, 33 and34 would not be required.

For any given sequence of transmissions, there are many Ways ofconstructing a receiver to give identical or equivalent information. Anumber of different circuit arrangements for giving phase differenceindications are shown in Figure 8 which is a block diagram of anotherform of receiver. The receiver of Figure 8 for simplicity is shown onlyas a receiver for indicating a single line of position. The receiver isassumed to be operating with a transmitting system employing frequenciesof f, 8f and 9i. The transmitting system will be assumed to operate on acycle in which there is a first period (e. g. 30 seconds) during which asignal of 6f is transmitted from a station A and signal of 8f istransmitted from a station B. For a short interval (e. g. half a second)these transmissions are interrupted and signals of 6f, 8f and 9i areradiated from station A and no signals from station B. The normaltransmissions are then resumed with 6i from A and 8f from B for afurther period (e. g. 30 seconds) and these transmissions are againinterrupted for a short period (e. g. half a second) during which nosignals are radiated from A and signals of 6], 8f and 9J are radiatedfrom B. The normal transmissions of 6j from A and 8f from B are thenresumed following the cycle just described.

Referring to Figure 8, there are provided amplifiers 46, 47 and 48 foramplifying respectively the signals of frequencies 6f, 9j and 8f whichIare picked up by a receiving antenna 49. The output of the amplifier 46is applied to one input of a phase discriminator 50 and the second inputof this phase discriminator is fed from an oscillator 51. The directvoltage output of the discriminator 50, which output depends on thephase relationship between the two 6j input signals, is applied to theoscillator 51 to control the frequency of the oscillator so as tomaintain a zero voltage output. Thus the two inputs from thediscriminator 50 are locked in phase. During the transmission periodsduring which the normal 6f transmission from station A is interrupted,the output of the discriminator 50 is prevented from controlling theoscillator 51 by a switching device 61 described hereinafter, which isoperated in synchronism with the switching of the transmissions. Y

In a similar manner the output of the 8f amplifier 48 is applied to oneinput of a discriminator 52 and the second input of the discriminator isfed from the output of an amplifier 53 tuned to a frequency of 8f. Thisamplifier 53 is fed with a signal of frequency 8f derived from a lfoscillator 54 through a lf pulse forming circuit 55 which produces aseries of harmonics in fixed phase relationship with the lf output ofthe oscillator 54. In some systems, the frequency 1f may be too low forconvenient use of a crystal oscillator and in that case the oscillator54 may be in the form of a beat oscillator using a pair of crystals. Theoutput voltage from the discriminator 52 controls the oscillator 54 soas to ensure that the two inputs to the discriminator are locked inphase. During the periods in which the 8f transmission from the stationB is interrupted, the output of the discriminator 52 is prevented fromcontrolling the oscillator 54 by the aforementioned switching device 61.

The circuit thus far described has two continuously operatingoscillators 51 and 54 which provide signals of frequencies 6f and 8fphase locked to the non-continuous received signals of thesefrequencies. These continuous signals from the oscillators areparticularly suitable for operating divider circuits as they are freefrom interference and from rapid phase shifts which might cause notchjumping of the dividers.

The 6f signal from oscillator 51 is applied to-the input of a divider 56which divides the frequency by six to give a 1f output. This output isapplied to an Vamplifier 57 which in turn feeds one mput of the phasedifference indicator 58. In a similar manner the 8f signal from theamplifier 53 is divided by the frequency divider 59 to produce a 1foutput which is fed to the amplifier 60 where it is amplified beforebeing applied to the other input of the indicator 58.

The aforementioned discriminator 50 associated with the 6]c signal fromstation A is of the double output type, that is to say it provides afirst direct voltage output which is proportional to the sine of thephase difference of the applied input signals and a second directvoltage output which is proportional to the cosine of the phasedifference. When the sine output is used to control the oscillator 51,that output is maintained at zero. The cosine output, therefore, is atits maximum positive or negative value. These two voltage outputs fromthe discriminator 50 can be utilised for synchronising the switching atthe receiver with that at the transmitter. For this purpose theswitching at the transmitters is synchronised in time with very shortduration alterations in phase of the radiation of the f signals fromstation A. When the radiation of frequency 6f is altered in phase,resultant direct voltages are produced during these signalling periodsby the discriminator 50 at the receiver and are applied to a signallingand switching circuit 61 which controls the timing of the switching atthe receiver. The duration of the phase-changed signalling transmissionsis made so short that it does not affect the indicators at the receiver.

The outputs from the amplifiers 46, 47 and 48 are combined and fed intoa coincidence detector 62. The phase changes through the amplifiers 47and 48 are adjusted so that, when the three amplifiers are fed withreference signals in known phase relationship, the input to thecoincidence detector has the Waveform shown in Figure 2. The coincidencedetector (a circuit which indicates the time at which the positive-goingpeaks of several combined signals are coincident) delivers pips at afrequency of lf to a pip amplifier 63 which in turn notches the dividers56 and 59 at the appropriate time as controlled by switches 64,and 65.

A fine pattern of phase difference indication is obtained by comparingtwo signals at a frequency 24J". One of the 24Jc signals is obtainedfrom a 241 amplifier 66 which amplifies the twenty-fourth harmonicderived from the lf pulse output of divider 56. The other 24f signal isobtained from an amplifier 67 which amplifies signals of frequency 24]cderived from the oscillator 53, the output of the oscillator 53 for thispurpose being amplified by an amplifier 68 and multiplied in frequencyby a frequency multiplier 69. The phase difference between the outputsof the amplifiers 66 and 67 is indicated by a phase indicator 70.

An intermediate pattern of phase difference indication based oneffective 8f transmissions from the two stations is obtained by mixingthe 6j signal from oscillator 51 with the 8f signal from amplifier 53 ina mixer 71 to give a 2f output signal which is applied to one input of aphase difference indicator 72. The output of divider 56 is multiplied bytwo in a multiplier 73 whose 2f output is amplified by an amplifier 74to provide a second input signal to the phase indicator 72.

A reference generator for standardising thereceiver is provided by theoscillator 54, the output of which after passing through a phase shifter75, is fed to a pip generator 76 which generates pips at a recurrencefrequency of lf thereby providing a series of harmonics Vof thefundamental frequency f in fixed phase relationship.

Y Yetanother form Yofrreceiver` making use of the information obtainablefrom the radiation of a plurality of signals from one station is shownin Figure 9. This receiver is for use in a four station navigationYsystem having a central master station and three slave stations. Theseslave stations, forconvenience, will be termed the G, Rand? slaves. Inthis example, thenormal master frequency is assumed to be 96 kc./s.,thenorma1 P slave frequency 80 kc./s., the normal'R slave frequency 128 fslavechannel.

Y 9 kc./vs. and the normal Gslave frequency .144,kc./s.4 `It will Vbe,seen thatA these frequencies are the siXth, fifth, eighth and ninthharmonics of a l6.kc./s.rfundamental frequency.v yDuring normaltransmissions, a single frequency is Yradiated from each station andthese signals are employed in the manner described in British patentspecification No. 620,479, the non-ambiguous pattern frequencies being288 ,kc/s. for the G and masterjpair, 384 k'c./s. for the R andmasterpair and 480 kc,/'s. for the P and master pair. The receiver ofFigure 9, as will be described later, is. arranged .also to producenonambiguous indications for each of these pairs with respect to patternfrequencies of both 16 kc./s. and 1 kc./s. For this purpose each of thetransmitting stations in turn are arranged to radiate simultaneouslyfrequencies 'of 80 kc./s., 96'kc./s., 128.kc./s., 144 1go/s., 143 kc./s.and 140 kc./ s.,. the radiations of this group of signals from eachstation being for a period of half of a second in each minute. Thisswitching of the transmissions is synchronised with some modification ofthe.` transmission for signalling purposes so that switching at thereceiver may be synchronised with the switching of the transmissions.This switching synchronism may be effected in the same manner as hasbeen described with reference to FigureS or in anyA other convenientmanner. As previouslymenti'oned, many methods for the controlof'switching at the receiver Vin synchronism with switching at thetransmitter are known in the art.

'Referring to Figure .9, there are shown six separate amplifiers 811-85tuned respectively to frequenciesrtof 80 kc./s., 96 kc./s., 128 lo/S.,144 kc./s., 143 kc./s. and 140 kc./s. The inputs to all these amplifiersare coupled to a common antenna 86 which receives all the radiatedsignals. vThe output of the amplifier 81 is yapplied to one input'of avphase `discriminat'or 87 and to the otherr input of this phaseYdiscriminator is fed the output of a 96 lso/Vs. oscillatorv 88.V Thedirect voltagevoutput of the discriminator is Y,applied to theoscillator 88 so as vto control the Yfrequency thereof in suchmannerthat the output ,of the oscillatoraj is loclged in phase to theoutput `of the amplifier 81Yduring the periods when the 96 .kc/s.`signal y. islradiatedfrom themasterlstation. 'In a similar manner anMSVOkc./,s.oscillator.,89 is' 'phase locked to theV normal transmission fromthe P slave by means of anph-ase 'discriminator 90. A 128 kc./s.oscillator 91 isphase locked to the unormal transmission from the Rslaveby means of avphase discriminator 92 and a. 144,kc./s; oscillator 93 isvk'phase locked to the/normal transmission from the G Slave by means ofadiscriminatorv94. l l Y The 96 kc./s. output of oscillator 88 isapplied to a frequency .divider 95 which divides the frequency by liveto, produce a 16 kc./s. output signalwhich is applied .to anA amplifier97.,.The output of the amplifier 97 isv ap-Y plied to another frequencydivider '98A which dividesthe r,frequency/to produce a l kc.,/s. outputsignal which is Afedftowangampliiier 99. .Similarly 'the output of `the8 ()k c./s. oscillator89 isrdivided in frequency byve in aQfiequencydivider 100 to give a 16 ykc./s. signal whichr 'isamplified in anamplifier 101 y and which is fed to a frequency divider 102lto produce a1 kc. /s. output applied to'v anamplifier v103,.V The 128y 1go/s.V.output of oscillator `911s divided by eight in the frequency divider103 -tot producea 16 .ko/s., output ywhich kis amplifiedin'll''anamplier .184j and which 'is fed to a frequency divider 16S' toproduce a1 vlic/s. output applied Vto amplilier ,106. Likewise the y144lkc/ s. output lof'foscillator 9 3 is` applied to a frequency divider 107to produce a 16 kcA/s. output which is amplif'edin an amplifier 108 andfed to a fnrtherl frequency divider 109 which produces a' 1 kc./s.signal, which in turn is applied to 4an :amplifier 110', The circuitsassociated withthe amplifierl 81 constitute a mas-` ter stationchannel,.those associated with` amplifier r8,0 a P t Phase indicatorsV111, 112 and 113 indicate `the phase difference between the 16 lio/s.signal in the master channel and each of the three ,16 lie/s. signals`in the Pslave, R slave and G slave channels respectively. v.Similarlyphase indicators 114, 115 and 116 indicatel respectively the phasedifference between the l kc./s. signal in the master channel and each ofthe three 1 kc./s. signals in the three slave channels. s

It Will be appreciated that if the indicators 111-113 are to indicateunambiguously position lines with respect to 16 l c./s. patterns and ifthe indicators 114-116 are to indicate position lines unambiguously withrespect to 1 kc./s. patterns, then the dividers in the master and slavechannels must be properly not-ched. The dividers 95,

118 will have a wave form, such as is shown in Figure 3,`

consisting of the fifth, sixth, eighth and ninth harmonics of a 16kc./s. fundamental frequency. The notching output signal of theamplifier 117 has to be switched on to the appropriate divider insynchronism with the trans- K missions so that the divider in eachchannel is synchronised by the signal derived from the simultaneousradiation of fourksignals from the associated transmitting station. p

The combined signals from all the ampliiiersStl-Sare applied to acoincidence detector 121) which feeds a pulse amplifier 121 to producepulses having a 1 kc./s.j repetition rate. The output of the amplifier121 isV switched on to the dividers 98, 192, and 109 in turn lat theappropriate time intervals when the six signals are radiated by theassociated station. This output serves to notch these dividers.

The output of the masterroscillator 88v is applied to the v inputs ofthree frequency multipliers 122, 123 and .124 to K produce signals inamplifiers 125, 126 and 127 having frequencies of 288, 384 and 480kc./s. respectively. Y A second signal of `frequency 288 kc./s. isproduced by multiplying Vthe output of oscillator 93 in a frequencymultiplier 12,8 and is amplifiedA in an amplifier 129'. A phaseindicator 134) is provided to indicate the phase'relationship of the 288lic/s. outputs from amplifiers 125. Similarly a 384 kc./s.'signal isproduced by multiplying the output of oscillator 91 in a frequency`multiplier 131 and this signal `is amplified in an amplifier 132 beforebeing applied to one input of the phase difference indicator 133 which4compares the phase of this signal with the output of amplifier 126. Asecondl output of 480 kc./s. isobtained by multiplying the outputofoscillator and 136 each serve to provide position line information fromythe, normal transmissions from the master and the three slave stations.rThese indications are not affected by Yany 'short interruptions of theradiated signals sincel Y the indications are obtained from theoscillators phase locked to the received signals during the normaltransmissions. The indicators 13),A 133 andf136 provide fine patternAindications which maybe ambiguous but the remaining indicators 111-116provide intermediateand coarse pattern indications toelirninate `orreduce the am-y biguities in the fine pattern indications. l

In the systems describedjwith reference to Figures 8 and 9, ,a num`berof signals are radiated simultaneously from each station. It will beappreciated that if these signals are combined in af/rnulti-tunednantenna and .if `the phaserelationship of `these signalsis similar tothep'hase 1'1 relationship of the harmonics derived from a pipgenerator, then the wave form would be peaked -as shown in Figures 2 and3. lt is particularly convenient to use a pip generator for referencingthe receivers and the receivers, therefore, are referenced on the basisof the signals being radiated in the phase relationship such as wouldgive this peaked wave form. However, in order to avoid the excessivevoltages on the antenna consequent upon the use of such a peaked waveform, an arbitrary phase change may be made in one or more of theradiated signals and equal changes introduced in the receiver. Forexample, in an arrangement employing fifth, sixth, eighth and ninthharmonics, the fifth harmonic may be reversed in phase at thetransmitter and a similar phase reversal is effected in the receiverafter the receiver has been referenced by means of the reference signalgenerator. A phase reversal can readily be effected accurately simply byreversing a pair of connections to a transformer and hence need notaffect the accuracy of referencing.

We claim:

1. A hyperbolic radio navigation system in which a position line isdetermined by indicating the difference in the time of propagation to areceiver of signals emitted from two stations in known spacedgeographical positions, wherein three signals are emitted from a rststation satisfying the respective phase conditions, given in radians ofangle, nlwUt-i-al, nzwO-l-az and (nl-l-lwot-i-a, and three signals areemitted from the second station satisfying the respective phaseconditions nlwOt-l-k, nzwut-lraz-f-k and (nl-l--UwOt-l-aS-f-k, where nland n2 are integers, wo is 21r multiplied by a fundamental frequency incycles per second, t is time in seconds and a1, a2, a3 and k areconstants, the transmissions being switched in sequence such that thesignals of each frequency are transmitted from one station duringintervals of the tranmissions of the same frequency from the otherstation, and the transmission being distinctively altered periodicallyfor-synchronising switch-ing means at the receiver with the switching ofthe transmissions and wherein the receiver comprises means for receivingthe radiated signals, switching means which are synchronised by thedistinctive alteration in transmission and which are arranged toseparate the received signals from the two transmitters, and a phasedifference indicator for indicating the difference in time ofpropagation which indicator has a recurrence cycle of time differenceindication equal, in seconds, to the reciprocal of the fundamentalfrequency, said indicator being operated 'by a tine control and a coarsecontrol, said line control being dependent on a pair of signals derivedfrom locally generated non-interrupted signals, one of which isphase-controlled by only one of the signals received from said firststation and the other by only one of the signals received from saidsecond station and the coarse control being dependent on six receivedsignals derived from the three different frequencies transmitted fromeach of the stations.

2. A hyperbolic radio navigation system as claimed in cla-im 1 whereinthe two locally generated non-interrupted signals are produced in thereceiver by a pair of oscillators arranged to oscillate at the samefrequency, which frequency is one of the radiated frequencies, the twooscillators being controlled in phase respectively by the signalsreceived from the two stations.

3. A hyperbolic radio navigation system as claimed in claim l wherein atleast one of the two locally generated non-interrupted signals isproduced in the receiver by an oscillator arranged to oscillate at saidfundamental frequency.

4. A hyperbolic radio navigation system as claimed in claim l wherein,at each station, the various different frequencies are radiated insequence.

5. A hyperbolic radio navigation system as claimed in claim l wherein,at each station, the various different frequencies are radiatedsimultaneously.

6. A 'hyperbolic radio navigation system as claimed in claim 5 whereinthe intermittent radiation of the plurality of different frequencies ateach of the two stations occurs during intervals in a normaltransmission in which signals of different frequencies are radiatedsimultaneously from the two stations.

v 7. A hyperbolic radio navigation system as claimed in claim 6 wherein,during the normal transmission, a single signal is radiated from eachstation, the frequencies of each of the signals being one of thefrequencies n1, n2 or nl-i-l multiplied by the fundamental frequency.

8. A hyperbolic radio navigation system as claimed lin claim 2 wherein,at the receiver, the simultaneously received signals of differentfrequency from the first station are combined to produce a signal of thefundamental frequency, which signal of the fundamental frequency is usedto phase-lock a frequency divider coupled to divide, to the fundamentalfrequency, that one of the locally generated non-interrupted signalswhich is phase-controlled by the first station therebyV providing a rstoutput of said fundamental frequency and wherein the simultaneouslyreceived signals of different frequency from the second station arecombined to produce a signal of the fundamental frequency, which signalof the fundamental frequency is used to phase-lock a second frequencydivider coupled to divide, to the fundamental frequency, the second ofthe locally generated non-interrupted signals which are phase-controlledby the second station thereby providing a second output of saidfundamental frequency and wherein said coarse control is effected 'byphase cornparison of the two output signals of the fundamentalfrequency.

9. A hyperbolic radio navigation system as claimed in claim 2 wherein,at the receiver there are provided a pair of dividers associatedrespectively wih the two oscillators, each divider being arranged todivide the oscillator output frequency rto a frequency equal to thedifference between n1 and n2 multiplied by the fundamental frequency andwherein each of the dividers is phase-locked by a beat signal obtainedby combining two signals one of which is the output of the associatedoscillator and the other Iof which is that one of the three signalsreceived from the transmitter controlling the associated oscillatorwhich differs in frequencies from the oscillator frequency by an amountequal to the frequencies of the divider output.

10. A hyperbolic radio navigation system as claimed in claim 9 whereinthe oscillators at the receivers are controlled by the received signalsof frequency n1 or nl-i-l multiplied by the fundamental frequency andwherein there are provided a further pair of frequency dividers arrangedrespectively t-o divide the outputs of the .rst pair of dividers to thefundamental frequency and wherein each of the second pair of dividers isphase-locked by a 'beat signal obtained by combining two signals one ofwhich is the output of the associated oscillator and the other of whichis that one of the three signals received from the transmittercontrolling the associated oscillator which differs in frequency'fromth-e oscillator frequency by an amount equal to the fundamentalfrequency, the

two outputs from the further pair of dividers being used to effect thecoarse'control of the phase difference indicator.

11. A hyperbolic radio navigation system as claimed in claim 1 whereinthere is provided a third transmitting station radiating signalssatisfying the respective phase conditions given in radians of angle,nlwct-l-al-l-kl, nZwOt-i-aZ-l-kl and (n1-l-1)w0tl-a3+k1 where k1 is aconstant, the transmissions being switched in sequence such that thesignals of each frequency are transmitted from one station duringintervals of the transmissions of the same frequency from the otherstations, and wherein the receiver is provided with two of said phasedifference indicators, one indicator being arranged to indicate thedifference in time of propagation of signals from the first and thesecond stations and the other being arranged to indicate ,the differencein time of als fromy the first and 'third stations. Y Y

12. A hyperbolic radio navigation system in which, at a mobile receiver,a yphase comparison is made between signals' derived from transmissions`from a pai'rof spaced stations, which system comprises a masterstationhaving means for intermittently radiatingia plurality of signals ofdifferent frequency in fixed multiple phase relation, thefrequencydiiference of two of the radiated signalsbeing asubmultiple ofall the frequencies, a slave station 'spaced from the master station andhaving means for intermittently radiatingsignals of the same frequenciesas are-radiated by the master station and in fixed phaserelation'therewith, Ythe signals of each frequency being radiatedfromrthe slave station during the'intervals between radiation of signalsof corresponding frequency from the master ,statiomand a mobilereceiver, which receiver comprises means for receiving andseparating'the signals from the master` andV slave stations, a pair ofsources of locally generated signals, means for synchronising theoutputof One of the sources in phase and frequency with received mastersignals of one frequency, means for synchronising the output of theother source in phase land frequency withreceived vslave signals of saidone frequency, a lirst frequency dividing means for dividing the outputof theiirstjsource toV said sub-multiple frequency, meansfortphase-locking the first dividing means utilising the signalsreceived from the master station, a second frequency dividing means fordividing the output 'of the propagation of sigsecond source to saidsub-multiple frequency, means for phase-locking the second dividingmeans utilising the signals received from the slave station and a phaseindica- `torrforv indicatingthephase relationship of the outputs fromthetwo dividing means v13. A hyperbolic radio navigation system having apair of spaced transmitting stations and a mobile receiver,

v one of said stations having means for intermittently radiating threesignals of different given frequencies in fixed multiple phase relation,the other of said stations having means for intermittently radiatingthree signals of said given frequencies in fixed multiple phase relationwith the signals radiated by said one station, means at said firststation for effecting a distinctive periodicrsynchronising transmission,switching means at said other station actuated by said synchronisingtransmission from said one station to control the timing of thetransmission from said other station, the timing of said transmissionsbeing such that within each period each station transmits a signal ofeach frequency and only one station, transmits a signal of a givenfrequency at a given time; and said -mobile receiver having means forseparately receiving signals yof said three frequencies, means forproducing a first and second unmodulated signal having a frequency squalto the lowest dierence frequency of the received signals, means forphase control of said first and second unmodulated signals such as toproduce a phase difference representativeof the difference in distanceto said two stations comprising a first fine phase control operativeover a fraction of a cycle applied to the first unmodulated signal forholding a line phase relation to one of the received signals from saidone station, asecond tine phase control operativeover a fraction of acycle applied to the second unmodulated signal for holding a given phaserelation to one of the received signals, from said' other station, acoarse phase control to remove any fractional ambiguity ,with a cycleapplied to said first unmodulated signal and derived from a phaserelation involving all three signals received from said one station, anda coarse phase control applied to the second unmodulated signal andderived from a phase relation involving all three signals received fromYsaid Vother station.

,14. A radio navigation system comprising: two spaced transmittingstations and a mobile receiver, means to derive a iirst, second, thirdand fourth non-interrupted --14 t signal Yof given frequency, meansV atone of said stations to transmit three interrupted signalshaving'frequencies which are differentmultiples of said given frequencyand having a fixed multiple phase relation to said firstV Signal, meansat the other of said stations to transmit three interrupted. signalshaving frequencies identical to those transmitted from said one stationVVand having a fixed multiple phase relation to said second signal ofgiven frequency, means for eifecting a cycle of switched transmissionsin which all three frequencies are transmitted from each station and nosignal of 'the same frequency is transmitted from more than one stationat the same time, means at said mobile receiver for separately receivingand amplifying the receivedv signals of different frequencies, a fineand acoarse phase control Y means applied to the third and fourthnon-interrupted signals of given frequency, the tine phase control ofthe third non-interrupted signal being derived from one Vof the signalsreceived from said one station, and thecoarse phasel control of saidthird non-interrupted signal being derived from all three signalsreceived from said one station, the fine phase control of the fourthnon-inter-v rupted signal being derived from Vone of the'signalsreceived from said other station, Vand the coarse phase control of saidfourth non-interrupted signal being derived l from all three signalsreceived from said other' station,

and a phase dilference indicator giving the phase difference between thethird and fourth signals of given frequency and arranged to indicate thedifference in distance between the iirst and second stations.

l5. A radio navigation Vsystem having spaced transmitting stationseachprovided` with a Vcontinuous signal source vof given frequency, amobile receiver provided with ltwo other continuous `signalsonrces ofsaid given frequency, means to transmit three different harmonics ofsaid given frequency from both of said stations, means to provide afixed multiple phase relation between the signals of given frequency ata given'station and its radiated harmonics, means to provide a xedphaseV dilference between the two signals of given frequency atl the twostations, means at said mobile receiver forrproviding a given complexphase relation between all three signals received from one of saidstations and the given frequency of one of said two other sources ofgiven frequency and to provide the same given complex phase relationbetween all three signals received from the kother of said stations andthe given frequencies of the other of said two sources of givenfrequency, thereby providing'a phase difference between said othersources of given frequency which is indicative of the difference indistance between said two stations. f

16. A radio navigation system comprising: two spaced transmittingstations and a mobile receiver, means at one of said stations includinga source of non-interrupted signals of frequency FA toperiodically'radiate harmonically related signals of frequencies nlFA,nzFA, andnFA,

where u1, n2, and n3, are different integerstand FAV is the tfundamental frequency of the harmonic series, means at the other of saidstations including a source of non-- interrupted signals of frequence FBto periodically'radiate Y harmonically related signals of frequenciesnlFB, 'n2FB, and nBFB, where FB is equal to FA, means to maintain apredetermined multiple phase relation between saidy signal of frequencyFA and each of said signals of frequenciesv nlFA, nZFA, and naFA, andbetween said signal of frequency FB and eachof said signals offrequencies niFB, )tgl-TB,

of the other pair having frequencies FD and nbFp, -whereL na and nb areany onel of the integers n1, n2 and n3, and

wherein FC and FD are equal to FA, said receiver in.

15 i Y cluding means for receiving transmitted signals of fre-V quenciesnIFA, nzFA, n3FA, nlFB, nQFB, and nal-TB, means for maintaining apredetermined phase relation between the receiver signal of frequencynlFA and the non-inten rupted signal of frequency nlFC, and between thereceived signal of nlFB and the non-interrupted signal of MED, andbetween each of the three received signals of frequencies nlFA, nzFA,and n3FA and the non-interrupted signal of frequency FC, and betweeneach of the three received signals of frequencies nlFB, nZFB, and naFB,and the non-interrupted signal of frequency FD, the phase control ofsaid non-interrupted signals of frequencies FC and FD acting as a coarsecontrol and the phase control of said non-interrupted signals offrequencies nlFC and nlFD acting as a tine control, and a phasedifference indicator for measuring the phase difference between thenoninterrupted signals of frequencies FC and FD to produce a readingindicative `of the difference in distance from the receiver to the twostations.

17. In a hyperbolic radio navigational aid in which all signals areharmonically related, a first transmitter for radiating at least threesignals of different given frequencies, a second transmitter forradiating at least three signals of said different given frequencies,means for generating a rst non-interrupted sub-multiple frequency signalhaving a phase control by said three, at

least, signals separately received from said first transmitter, meansfor generating a second non-interrupted sub-multiple frequency signalhaving the same frequency as that of said first sub-multiple frequencysignal, and having a phase control operated by said three, at least,signals separately received from said second transmitter, meansproviding alternate synchronized transmissions from said rst and secondtransmitters so as to avoid dual t A 16 transmission of a singlefrequency, and a means to indicate the phase difference between therstand second non-interrupted signals.

18. A radio navigation system comprising: two spaced transmittingstations and a mobile receiver, means to derive a rst, second, third andfourth non-interrupted signal of given frequency, means at one of saidstations to transmit three interrupted signals having frequencies whichare different multiples of said given frequency and having a fixedmultiple phase ,relation to said rst signal, means at the other of saidstations to transmit three interrupted signals having frequenciesidentical to those transmitted from said one station and having a xedmultiple phase relation to said second signal of given frequency, meansfor effecting a cycle of switched transmissions in which all threefrequencies are transmitted from each station and no signal of the samefrequency is transmitted from more than one station at the same time,means at said mobile receiver for separately receiving and amplifyingthe received signals of different frequencies, means for deriving a rstsub-multiple frequency signal from said separately amplified signalsreceived from said one station, means for deriving a second sub-multiplefrequency signal from said separately amplified signals received fromsaid other station, and means including a phase controlled oscillatorfor deriving a phase difference indication from said first and secondsub-multiple frequency signals.

References Cited in the le of this patent UNITED STATES PATENTS

